A continuously variable transmission (CVT) is a mechanical device in which the torque transmission ratio is infinitely variable over the working range, by contrast to a conventional transmission in which there is a limited number of selectable torque transmission ratios. A CVT automatically changes the ratio as required by load and speed conditions, providing an increased torque under high loads at low speeds and yet controlling the rotation speed of the motor as the vehicle accelerates. It is commonly used in a wide range of vehicles, such as small cars or trucks, snowmobiles, golf carts, all-terrain vehicles (ATV) and scooters. A CVT is usually coupled to a motor, such as an internal combustion engine or an electric motor.
A conventional CVT comprises a driving pulley mechanically connected to the motor, a driven pulley mechanically connected to wheels or a track, and a trapezoidal drivebelt transmitting torque between the driving pulley and the driven pulley. The sides of the drivebelt are, on each pulley, gripped between two opposite flanges that are coaxially mounted around a main shaft. One of the flanges is axially movable with reference to the other. Each flange is directly or indirectly in a torque-transmitting engagement with the corresponding main shaft.
Initially, such as when the vehicle is stopped or at low speeds, the winding diameter of the driving pulley is minimum and the winding diameter of the driven pulley is maximum. This is referred to as the minimum ratio since there is the minimum number of rotations or fraction of rotation of the driven pulley for each complete rotation of the driving pulley.
The driving pulley generally comprises a centrifugal mechanism that is provided to increase the ratio when its rotation speed increases. To do so, the centrifugal mechanism is able to apply a force on the movable flange of the driving pulley to move it closer to the fixed flange, thus urging the drivebelt to wind on a larger diameter around the driving pulley. At the same time, the shift in the position of the drivebelt towards the driving pulley urges the movable flange of the driven pulley away from the fixed flange thereof.
The driven pulley of a CVT is torque-sensitive. This allows the driven pulley to counterbalance the force generated by the centrifugal mechanism of the driving pulley so as to allow the motor speed to rise to an optimum level before the CVT starts upshifting during an acceleration. The driven pulley also allows the CVT to downshift if the load increases. Accordingly, the driven pulley comprises a cam system urging the movable flange to move towards the fixed flange of the driven pulley when the torque increases, thereby pulling back on the drivebelt and fighting the force generated by the centrifugal mechanism of the driving pulley. A conventional cam system comprises a cam plate having a plurality of symmetrically-disposed inclined cam surfaces on which respective cam followers are engaged. The cam followers are generally slider buttons or rollers. The cam plate or the set of cam followers is mounted at the back side of the fixed flange and the other of them is usually rigidly connected to the main shaft.
In use, the movable parts of the CVT constantly seek to rearranged their position until an equilibrium is reached or until they reach the maximum ratio. The ratio at which the CVT stabilizes is an equilibrium between the forces on the drivebelt applied by the driving and the driven pulley. At the maximum rotation speed, the ratio is maximum as there is the maximum number of rotations or fraction of rotation of the driven pulley for each complete rotation of the driving pulley. Then, when the rotation speed of the motor decreases, the force generated by the centrifugal mechanism decreases as well. Return springs located in the driving and in the driven pulley allow the corresponding movable flanges to move back to or near their original low ratio position.
A conventional centrifugal mechanism of a driving pulley generally comprises a set of centrifugal flyweights pushing their way through a pair of opposite inclined ramps converging towards the periphery of the driving pulley. Each of these flyweights are subjected to a centrifugal force F as in the following equation:F=m rω2 sin θwhere m is the mass of the flyweight, r is the radius from the center of the main shaft, ω is the rotation speed and θ is the angle of the ramps with reference to the main shaft. As one can see from the equation, the force is a function of the square of the rotation speed, which means that the centrifugal force increases more rapidly that the proportional increase in the rotation speed itself. Also, the flyweights are moved away from the center of the main shaft when the centrifugal force increases, which in turn also increases the force since the latter depends on the radius r. It follows that the centrifugal system of the driving pulley soon becomes proportionally stronger than the cam system of the driven pulley, thereby shifting the position of the drivebelt towards the driving pulley. As a result, a conventional CVT tends to upshifts too early towards the maximum ratio when the rotation speed of the driving pulley increases. This is partially kept under control by changing the angle of the ramps in function of the position of the flyweights, thus in function of the ratio. The angle of the ramps with reference to the axis of rotation is smaller at a higher ratio.
One of the drawbacks of a conventional driving pulley is thus the lack of direct control on the force generated by the centrifugal mechanism. Changing parameters such as the mass of the flyweights or the profile of the ramps to allow a higher motor speed during an acceleration is not always a suitable or possible solution due to the impacts it has on the overall behavior of the CVT. For instance, if the CVT is designed to allow a low rotation speed of the motor at a moderate cruising speed to reduce fuel consumption and noise, then during an intense acceleration, the rotation speed of the motor will most probably be too low. Conversely, if the CVT is designed for allowing a high rotation speed of the motor during an intense acceleration to obtain a maximum power therefrom, then the rotation speed is likely to be too high at moderate cruising speed.